There are two basic faceplate panel contours utilized commercially for rectangular CRT's having screen sizes greater than about a 23 cm diagonal: spherical, and cylindrical. Although flat contours are possible, the added thickness and weight of the faceplate panel required to maintain the same envelope strength are undesirable. Furthermore, if a flat faceplate CRT is a shadow mask color picture tube, the additional weight and complexity of an appropriate shadow mask also are undesirable.
Recently, it has been suggested that spherically-shaped CRT faceplate panels be improved by increasing the radius of curvature of the panels by a factor of 1.5 to 2. Such increase in radius of curvature reduces the curvature of the faceplate panel, thereby permitting more satisfactory off-axis viewing of a tube screen. Although such tubes having increased radius of curvature do provide improved viewing, there is still a need for even flatter faceplates or, alternatively, for tubes that appear to be flatter.
A new faceplate panel contour concept which creates the illusion of flatness is disclosed in three recently-filed, copending U.S. Applications: Ser. No. 469,772, filed by F. R. Ragland, Jr. on Feb. 25, 1983 and now U.S. Pat. No. 4,839,556; Ser. No. 469,774, filed by F. R. Ragland, Jr. on Feb. 25, 1983 and now U.S. Pat. No. 4,786,840; and Ser. No. 469,775, filed by R. J. D'Amato et al. on Feb. 25, 1983 and now abandoned. The contour has curvature along both the major and minor axes of the faceplate panel, but is nonspherical. In a preferred embodiment described in these applications, the peripheral border of the tube screen is planar. In such tubes, it is important to contour the faceplate panel diagnosis so that the differing curvatures extending from the major and minor axes are properly blended. In the above-cited U.S. application Ser. No. 469,774, this blending is accomplished by permitting at least one sign change of the second derivative of the diagonal contour in the center-to-corner direction.
The present invention provides a novel faceplate panel contour which appears flatter than the suggested longer radius tubes and which does not require the use of much thicker glass to maintain tube strength.